On FirefoxReality we increate mPresentingGeneration value to exit from WebVR from a back button action. This normally works but I did some tests with webpages that don't call vrDisplay.SubmitFrame and Exit from WebVR didn´t work ok.
The problem is that mPresentingGeneration changes are only checked in VRDisplay::SubmitFrame(). We should add a listener dispatch by the watchdog also to protect from situations where vrDisplay.SubmitFrame() is not called.